I. Field
Generally, the technical field involves controlling volatile organic compounds (VOCs) that are produced during microbial fermentation processes. Specifically, it is a method of reducing the concentration of VOCs emitted into the air during ethanol production.
II. Background
Fermentation is a process where microorganisms convert carbohydrates, such as sugar, into alcohols or acids. Microorganisms, such as yeast, fungi and bacteria, are used to produce a number of fermentation products, such as industrial grade ethanol, distilled spirits, beer, wine, pharmaceuticals and nutraceuticals (foodstuff that provides health benefits, such as fortified foods and dietary supplements). The microorganisms are combined in an aqueous solution with fermentable sugars. The microorganisms consume the sugars, converting them into aliphatic alcohols, such as ethanol.
The fermentation process produces a great deal of carbon dioxide that evolves from the top of the fermentor. In addition to carbon dioxide, other problematic gases are produced in the fermentation process. These gases are often classified as volatile organic compounds (VOCs). VOC emissions are regulated by the government, which means fermentation facility can only emit a limited quantity of VOCs into the air. The two primary regulated VOCs are acetyl aldehyde an acrolein. Other VOCs compounds are produced in minimal quantities or not regulated at all. Examples of these other VOCs include, ethyl acetate, ethanol, isoamyl alcohol, acetic acid, furaldehyde, formaldehyde, and formic acid. In response to the regulations discussed above, fermentation facilities, such as ethanol plants, attempt to limit or reduce their VOC output.
Most facilities currently employ a carbon dioxide scrubber to capture the carbon dioxide and VOCs exiting the fermentor(s). The carbon dioxide scrubber flows a water solution counter current to the gases at a rate between 25 to 300 gallons per minute.
This solubilizes the gases in a water solution. Reducing agents, such as sodium bisulfite or ammonium bisulfite, are then injected into the water solution. These reducing agents react with the VOCs to reduce the regulated compounds into non-regulated compounds. Chlorine dioxide (ClO2) has many industrial and municipal uses. When produced and handled properly, ClO2 is an effective and powerful biocide, disinfectant and oxidizer.
ClO2 has been used as a disinfectant in the food and beverage industries, wastewater treatment, industrial water treatment, cleaning and disinfections of medical wastes, textile bleaching, odor control for the rendering industry, circuit board cleansing in the electronics industry, and uses in the oil and gas industry. It is an effective biocide at low concentrations and over a wide pH range. ClO2 is desirable because when it reacts with an organism in water, it reduces to chlorite ion and then to chloride, which studies to date have shown does not pose a significant adverse risk to human health.
ClO2 is, however, unstable in the gas phase and will readily undergo decomposition into chlorine gas (Cl2), oxygen gas (O2), and heat. The high reactivity of ClO2 generally requires that it be produced and used at the same location.
Recently, it was discovered that chlorine dioxide effectively reduce undesirable microorganisms during propagation, conditioning and/or fermentation while encouraging propagation and/or conditioning of the desirable microorganisms and increase their efficiency in fermentation. This is discussed in co-owned U.S. patent application Ser. No. 11/626,272, filed Jan. 23, 2007, entitled “Apparatus and Method for Treatment of Microorganisms During Propagation, Conditioning and Fermentation,” which claims priority benefits from U.S. Provisional Patent Application Ser. No. 60/775,615, filed Feb. 22, 2006, entitled “Apparatus and Method for Treatment of Yeast During Propagation, Conditioning and Fermentation.” Both of these applications are hereby incorporated by reference in their entirety.
Accordingly, it would be desirable to provide a less costly and more effective method of oxidizing and thereby reducing VOCs produced during the fermentation processes.